When disputing anything with Equifax, you will have to write a letter and attach any proof you have. Online disputes do not work with Equifax for tricky stuff.
Do you have an old credit report that shows the OC account with the partial account number that you can compare to the first CA's account and the new CAs account? If you can prove on paper (and don't happen to have that last statement handy) that the DOFD should be a different date, the CRA would have to update their information.